When a phone call is placed from a radio telephone mobile station (MS) to a base station (BS), or when the best calling channel is searched for, in known cellular telephone systems presently in use the mobile station has to search a great number of radio channels according to acceptance criteria defined in the respective system specifications. For example in the NMT-900 system the number of channels to be searched is between 1 and 1999.
The channel search process in a mobile phone is described below using the NMT system as an example. FIG. 1 shows the blocks required for the channel search in an NMT telephone. The received signal is shifted to the intermediate frequency f.sub.IF in a mixer and then supplied to the intermediate frequency circuit 1 separating from the carrier the alternating speech signal and the signalling, which is supplied to the data demodulator 3. Here the signalling is converted into a binary signal and supplied to the microprocessor 5 of the telephone. When the data detector 4 discovers that there is signalling on the data line from the intermediate frequency circuit 1 it examines the activating pulse DDC. The intermediate frequency circuit 1 also supplies a d.c. voltage RSSI, which is proportional to the strength of the received high frequency signal. This d.c. voltage is sampled with the analog-to-digital converter 2 each time the converter receives a control pulse ADC. All information between the microprocessor 5 and the functional blocks is transmitted on the data bus. The channel search proceeds according to the timing diagram shown in FIG. 2, in which the operating times are shown as time related pulses. t.sub.0 is marked as the moment when the synthesizers receive a command to tune onto a new channel n. In the topmost timing diagram the time T.sub.0 shown as a pulse illustrates the time required to supply a new divisor to the synthesizer. This time is less than 1 ms. After the synthesizer settling time T1, about 10-15 ms, a short ADC pulse is supplied to the A/D-converter 2, whereby information about the prevailing strength of the received signal is obtained from the RSSI voltage. If the signal is below a given level, then a jump according to arrow A is made directly to a phase where the synthesizer is programmed to the next channel n+1. If the signal strength is above this given level, it is followed by an examination to see whether signalling is received from the intermediate frequency circuit 1. This is performed when an activating pulse DDC is supplied to the data detector 4. This examination takes the time T.sub.2 or about 2 to 3 ms. If no signalling is received, the synthesizer is immediately programmed to the next channel n+1. This jump is illustrated by arrow B. If received signalling is detected, there follows a waiting time T.sub.3, 0-10 ms, and then the received signal strength RSSI is again read by supplying the activating pulse to the A/D-converter 2. If the signal is too weak, then the synthesizer receives a command to change to the next channel n+1, jump C, but if the signal is acceptable, then the reception and interpretation of the signalling received from the base station is started. The time T4 allowed for the reception is less than or equal to 277 ms. This channel is kept, if according to the interpretation it is the channel searched for, otherwise the synthesizer is programmed to the next channel n+1.
This process is repeated in sequence for all channels in a given channel band, until an acceptable channel is found. There may be several search cycles before an acceptable channel is found. The channel is accepted on three conditions: first, the received signal must be above a given level. There are three levels: level A (20 dBmyV), level B (10 dBmyV), and level C (-2 dBmyV). First signals above the highest level are looked for, and if there are no such signals, a step to the next level is taken, and so on. Second, signalling must be received from the channel, and third, based on the interpretation of this signalling, the channel must be the channel looked for.
In present mobile phones a voltage controlled temperature compensated crystal oscillator is used as a frequency reference with an accuracy of the order 2 ppm, or at least of the order 10 ppm. For example in the GSM system the transmitter's carrier frequency stability must be below .+-.100 Hz or 0,1 ppm. Due to this frequency stability requirement the frequency must be controlled by an automatic frequency controller (AFC, Automatic Frequency Control).
In multichannel radio telephones such as in modern mobile phones the mixer frequencies of the receiver and the transmitter, i.e. the first and second local oscillator frequency of the receiver and the first and second shift oscillator frequency of the transmitter, are often formed by synthesizers from the radio telephone's own reference frequency. There may be one or more synthesizers to generate these signals. In a known way the synthesizers comprise a phase locked loop having an output frequency, which is locked to a reference frequency, whereby this reference frequency is another input of the phase comparator in the loop. The reference frequency again is generated by a voltage controlled, temperature compensated crystal oscillator, since without voltage control the frequency of a crystal oscillator manufactured with today's technique is not sufficiently accurate and the frequency stability is not sufficient, whereby the frequencies generated by the synthesizers would differ too much from the allowed, so that the frequency deviation from the carrier would be too large. E.g. the NMT-900 network allows a frequency deviation of .+-.0,8 ppm from the nominal frequency. Generally different automatic frequency control means (AFC) are used in order to control the frequency of the reference oscillator. The basic idea in the AFC methods is to lock the reference frequency in some way to the received carrier frequency, since the carrier frequency transmitted by a base station in cellular networks is very stable. According to the general principle the own first local oscillator frequency is compared with the received signal frequency, and the frequency of the own oscillator generating the reference frequency is controlled in accordance with the obtained frequency difference.
A modern AFC method is disclosed in the patent FI-79636, Nokia Matkapuhelimet Oy and the corresponding German Patent Application DE 3738124.5. There a constant time is generated with the aid of the frequency of the crystal oscillator connected to the processor, during which time the periods of a signal received and shifted to the second intermediate frequency are counted in a counter. The counting result is in digital form, and then after a D/A-conversion the counting result is used to control the reference oscillator. This mode, as well as many other modes utilizing the carrier frequency, are substantially based on the use of a processor performing the control and the calculations. Therefore the telephone's RF-parts require for the AFC only a buffer for the intermediate frequency signal, an output line for the intermediate frequency signal, and an input control line for the temperature compensated crystal oscillator. The AFC facility itself, or the calculation and the control, is performed by the processor and its peripheral equipment in the audio/logic parts of the telephone. The AFC facility moreover requires information about the received signal strength and a memory, which stores the correct control data or the frequency information for the temperature compensated crystal oscillator. It is necessary to store the frequency information regarding those situations where the signal received from the base station is momentarily lost (the maximum waiting time in the NMT system is about 4 minutes). In the NMT the AFC facility must not offset the output frequency more than 4 kHz from the nominal frequency, regardless of the frequency received from the base station. However, the control must be able to correct a possible error of 2,5 kHz, caused by temperature drift in the crystal oscillator. The advantage of known systems of this kind is that the frequency reference limits can be set by a program, and that the last control voltage value is left in the memory when the received signal fades out. On the other hand these known systems have a disadvantage in that they use the processor capacity by utilizing its program memory capacity. On the other hand this load is periodic. Further they require counters and at least the control lines mentioned above, as well as a plurality of connections between the RF-parts and the logic parts. The LFK patent application No 9203597.1 discloses an invention, with which it is possible to obviate the above mentioned disadvantages and to provide means to realize the whole AFC facility without programming, using only components (in hardware). Thus it is possible to realize all of the AFC facility in the radio frequency parts, whereby it is possible, considering the manufacture, to integrate the AFC facility on the same silicon chips as the other RF-parts, and thus to reduce the space requirements.
Due to the frequency jumps and the monitoring of the neighbour base station service area both the transmitter and the receiver must tune to a new frequency in less than one millisecond. This poses considerable requirements on the setting time of the synthesizer. Temperature variations and e.g. ageing will cause frequency drift. It has been tried to compensate frequency changes due to temperature changes by using an expensive voltage controlled temperature compensated crystal oscillator. On the other hand it is not so easy to compensate for frequency changes caused by ageing. The European patent application no.- 890111143.7 describes an AFC method operating according to the counter principle, with which it is possible to compensate for the frequency drift caused by ageing. Neither is the compensation possible with the method described in the publication, if the frequency provided by the reference oscillator differs largely from the correct value of the reference frequency.